Soap (Gr. σάπων, Lat. sapo), a compound formed by the union of alkalies with oils and fats. The invention of soap is ascribed by Pliny to the Gauls, and he gives the Germans credit for manufacturing both hard and soft soaps. From them the Romans learned the art, but soap was for a long time principally used by them as a wash for the hair. A complete soap-boiling establishment, and soap in a good state of preservation, have been discovered at Pompeii. Some natural productions possess the qualities of soap, as the berries of the soap tree (sapindus Saponaria) of South America and the West Indies, and the bark of the quillaja Saponaria, which has been carried from Peru to Liverpool for washing woollens. The juice of soapwort or bouncing Bet (Saponaria officinalis) forms a lather with water, and is used in England for scouring dresses. In California the roots of the pha-langium pomaridianum, which grows there abundantly, and has the odor of brown soap, is much used for washing clothes. Alkaline waters, when used upon greasy fabrics, form soaps similar to those produced in the regular manufacture.

Different kinds of oils may be used in soap making, having different proportions of the proximate principles of fatty bodies, stearine, palmitine, and oleine (see Oils and Fats), and also upon the kind of alkali, soda making a harder soap than potash. The hardest soaps are made with stearine and soda, the softest with oleine and potash. The natural combination of glycerine with the fatty acids is broken up by the action of the alkali, and the glycerine exists in a free state in the soap, or it may be extracted as a separate product. The principal fats and oils used for making soap are tallow, and palm, cocoanut, rape, poppy, linseed, hempseed, and olive oils; the last is used in the manufacture of the celebrated Castile, Marseilles, and other marbled and plain soaps of southern Europe. The best oils for marbled soaps come from Naples, and the Spanish oils are also highly esteemed. The oils from the East are not so rich in stearine, and are more or less colored green, which is objectionable. The mottling or marbling of soaps is produced by sprinkling the surface of the newly made body successively with lyes of less and less concentration, by which the soap is again rendered sufficiently pasty or semi-fluid to allow of the aggregation in different masses of the particles of coloring matter. - The ordinary method of saponification, as the conversion of fats into soaps is called, is by boiling them with solutions of caustic potash or soda.

Most fats require long continued boiling with excess of alkali, but others, as lard, beef marrow, and oil of sweet almonds, may be saponified by agitation with caustic alkali at ordinary temperatures; and under increased pressure the alkaline carbonates will readily produce saponification of fats. Rosin, which is capable of forming a soap with either potash or soda, is frequently added to soaps. Every kind of soap contains a variable quantity of water, partly in chemical combination. Soap is perfectly soluble in alcohol and hot water, but both solutions solidify to a jelly at a certain stage of concentration. Opodeldoc is soap mixed with alcohol in this state, to which camphor is added. Cold water does not dissolve the alkaline oleates, palmitates, and stearates which constitute ordinary soap, without decomposition, the alkali being dissolved and the oily acid precipitated; and when hot solutions are cooled the same action takes place. Soap is quite insoluble in a solution of common salt containing more than one part in 400 of water, so that on the addition of salt to the contents of a soap pan, a curd consisting of a solid soap will rise to the surface, while the alkaline salts and glycerine remain dissolved in the water.

Some soaps, as those made from cocoanut oil, are not so easily separated from their solutions by common salt. Other chlorides, as those of potassium and ammonium (sal ammoniac), have a similar action to that of common salt. Soaps are scented and colored by mixing coloring matter and volatile oils or odorous matters with them. They are sometimes medicated with antiseptic and other, substances, such as creosote, carbolic acid, chlorate of potash, and sulphur, and are used as detergents and in skin diseases. Arsenic is sometimes added to soap and used by taxidermists in preserving their preparations. Those medicinal preparations called liniments are soaps whenever they are made by the mixture of an alkali or an alkaline earth with an oil. Silicate of sodium (soluble glass) may be mixed with soap and used with advantage as a domestic cleansing agent. Soaps mixed with fine sand or pumice stone do not possess the same detergent properties, but are useful for scouring. The manufacture of soap is more largely carried on in Great Britain than in any other country, although great quantities of toilet soaps are made in France, especially for the American market. The annual product of Great Britain is often considerably over 200,000,000. lbs.

The manufacture is also carried on to a considerable extent in the United States, and some fine toilet and other soaps are made. - The history of soap may be found in Beckmann's "History of Inventions;" its technology in Parnell's " Chemistry applied to the Arts," Knapp's "Chemical Technology," Wagner's " Chemical Technology," Muspratt's "Chemistry," Morfit's "Applied Chemistry in the Manufacture of Soaps and Candles," and in "A General Treatise on the Manufacture of Soap," by H. Dussauce (8vo, Philadelphia, 1869). The French manufacture is described in one of the Manuels Roret entitled Nouveau manuel theorique et pratique du savonier, ou l'Art de faire toutes sortes de savons (Paris, 1852).